This invention relates to the art of induction heating and, more particularly, to an apparatus for inductively heating simultaneously a plurality of identical elongated workpieces respectively having a plurality of axial spaced portions of different cross-sectional area each of which is to be subjected to different heating levels and wherein each workpiece is to be heated to have the same heat profile extending along the length thereof.
The invention is particularly applicable to the heating of sucker rods which are employed in oil wells and the like and will be described in particular reference thereto. It will be appreciated, however, that the invention has broader application and may be used for inductively heating simultaneously and alike a plurality of identical bars having a uniform cross-sectional area throughout their length wherein the heat profile extending along the surface of the bar is to be uneven, i.e., the surface at one end of the bar is to be hotter than the other end.
Sucker rods are utilized in the petroleum industry as a connecting link between a down hole oil well pump and the lifting or pumping device on the surface. The rods are of generally cylindrical form and are quite long, normally being on the order of magnitude of 25 to 30 feet. A major portion of the rod has a uniform cross-section with an enlargement being included adjacent each end thereof. These enlargements facilitate interconnecting a plurality of the rods in an end-to-end relationship with each other. While there are a number of specific or detailed modifications which may be included in the sucker rod configurations of different manufacturers, almost all such rods have the foregoing general configuration and characteristics.
As one of the manufacturing steps, the sucker rods are heated identically to a given predetermined temperature and then passed through an electrostatic spray chamber for application of a coating of paint or plastic-like material thereto. Since the rods need only be surface heated for this purpose, induction type heating finds particular use in this environment. Also, it is generally desirable for economic reasons to heat more than one sucker rod at a time. This can be done by passing the several sucker rods simultaneously through sets of like inductor coils connected in series or in parallel to a suitable power supply, each set comprising a separate induction heating coil for each workpiece. With this type of heating arrangement however, it is essential that each individual workpiece enter its respective coil simultaneously. This is due to the fact that the circuit conditions must be changed during the progress of the sucker rod through the coil because of the diameter variations along the length of the rod. These mass variations in the rods require that the circuit conditions in the coils vary, as a sucker rod is passed through the coil, to insure uniform surface heating of the workpiece.